1. Field of the Invention
The present invention relates to a magneto-optical recording medium for recording and regeneration of information through the use of a laser beam using a magneto-optical effect. More specifically, it relates to a magneto-optical recording medium that uses a magnetic domain wall displacement due to a temperature gradient and can regenerate signals recorded therein with a high density over the limit of resolution of optical systems.
2. Related Background Art
As rewritable recording media, various magnetic recording media have been put into practical use. In particular, magneto-optical recording media, which records information by forming a magnetic domain in a magnetic thin film using thermal energy from a semiconductor laser and regenerates the information using a magneto-optical effect, are promising mass commutative media capable of high-density recording. In recent years, in accordance with the trend toward digitization of moving picture, it has been required to increase the recording density of the magnetic recording media to provide a recording medium with a higher capacity.
In general, the track recording density of optical recording media significantly depends on a laser wavelength and a numerical aperture NA of an objective lens in a regenerating optical system. In other words, the laser wavelength λ and the numerical aperture NA of the objective lens in the regenerating optical system determine the diameter of the beam waist. Thus, the spatial frequency of a recording pit that can regenerate a signal is limited to about 2NA/λ. Therefore, for increasing the recording density of conventional optical disks, it is required to reduce the laser wavelength of the regenerating optical system or increase the numerical aperture of the objective lens. However, the laser wavelength is difficult to reduce in terms of efficiency and heat generation of the element, and if the numerical aperture of the objective lens is increased, more severe mechanical precision becomes required because of reduction of the focal depth or the like.
Thus, various super resolution techniques have been developed which are improved in configuration of the recording medium and method of regenerating information and provide improved recording density without changing the laser wavelength and the numerical aperture of the objective lens. For example, in Japanese Patent Laid-Open No. 3-93058, there has been proposed a signal regenerating method in which a signal is recorded in a multilayer recording layer comprising a regenerating sub-layer and a recording sub-layer magnetically coupled to each other, the directions of magnetization of the regenerating sub-layer are aligned before the regenerating sub-layer is irradiated with laser light to be heated, and then the signal recorded in the recording sub-layer is read out by being transferred to a region of the regenerating sub-layer in which the temperature has been increased. According to this method, since the signal detection region can be limited within a desired region in which the temperature is increased, which is smaller than the regenerating spot, an intersymbol interference during regeneration of information can be reduced, and a signal of a spatial frequency equal to or higher than 2NA/λ can be regenerated. However, this method has a disadvantage that, since the available signal detection region is smaller than the spot of the regenerating laser, the amplitude of the regenerated signal is reduced, the resulting output power of regeneration is insufficient. Therefore, the available signal detection region cannot be significantly smaller than the spot size. Consequently, the recording density cannot be significantly increased compared to that determined by the diffraction limited of the optical system.
In view of such a problem, in Japanese Patent Application Laid-Open No. 6-290496, the present inventors have already proposed a magnetic recording medium and regenerating method in which a magnetic domain wall located at a boundary of a recording mark is displaced due to a temperature gradient toward a side of higher temperature, and the domain wall displacement is detected, whereby signals recorded therein with a high density over the limit of resolution of optical systems can be regenerated without reducing the amplitude of the regenerated signals.
In method described in Japanese Patent Application Laid-Open No. 6-290496, if the temperature gradient is to be provided by heating the medium using the regenerating light beam itself, the peak of the temperature distribution appears in the regenerating light beam spot. Accordingly, in a region having a temperature that allows the domain wall to be displaced, a domain wall displacement from a front end of the region and a domain wall displacement from a rear end of the region are both read out by the regenerating spot, and a satisfactory regenerated signal cannot be obtained. Thus, besides the regenerating light beam, it is required to additionally provide means for providing a desired temperature distribution. Therefore, there is a problem that the regenerating apparatus becomes more complicated. Furthermore, in Japanese Patent Application Laid-Open No. 9-235885, there has been proposed a method in which the domain wall displacement from the rear end is suppressed by applying a regenerating magnetic field. However, this method has a problem that a margin of the regenerating magnetic field that provide a satisfactory regenerated signal is narrow, and the resulting signal waveform is asymmetrical.
Furthermore, in Japanese Patent Application Laid-Open No. 2000-187898, there has been proposed a method in which the domain wall displacement from the rear end is suppressed by inserting a magnetic layer having a high domain wall energy density (control layer) to control a transfer condition in the rear part of the temperature distribution. This method has completely solved the above-described problem regarding essential operations. However, it is proved that, if a pit portion having irregularities for providing address information or servo information is provided on the substrate, controllability of the transfer condition or the like is lost due to a variation of deposition of a magnetic film onto the portion or the like, and thus, the domain wall is easily displaced from the rear end in the portion. That is, after the regenerating spot passes over the pit portion, the direction of magnetization of the portion behind the regenerating spot is inverted, and the regenerated signal largely varies in a DC manner. This phenomenon causes an edge shift of the regenerated signal and becomes a problem in assuring a margin for regenerating information or increasing the recording density.